抄録
It is well known that the strength and the modulus of elasticity of some materials are greater for rapid application of load than for slow loading. The theoretical interpretation of this fact has been given in the past based on the linear viscoelastic theory. The present experimental study was carried out to investigate the effects of strain or stress rate the compressive ultimate strength, the modulus of elasticity and the deformation characteristics of cement mortar and concrete at age of 1 to 28 days. The test variables were: the mean rate of stress, σ=0.01 to 35 (kgf/cm2·sec), the mean rate of strain, ε=0.15 to 450 (×10-6/sec); the test age of 1, 3, 7, and 28 days; the water-cement ratio W/C=0.45 to 0.75. 10φ×20cm size cylindrical specimens were used for all tests.
The test results were summarized as follows. The dependence of the ultimate strength on strain rate for concrete was more remarkable at an early-age than at the age of 28 days. On the contrary, this dependence for cement mortar was remarkable at the age of 28 days. The strain at the ultimate strength of concrete at an early-age was influenced by strain rate, but it was not influenced at 28 days. The coefficient of viscosity of concrete λ is expressed by the equation as λ=aε-b, and the relationship between stress rate and strain rate is given by σ=αεβ. The variation of concrete strength σu, which is brought by various rates of loading, can be expressed by the following equation based on the rheological theory, where σu, s is the statical ultimate strength.
σu=σu, s+λε(t)
